The cathode of the electron gun of an E. B machine is usually a tungsten wire or band cathode, heated directly by passing a heating current through it, the operating temperature of the cathode being a direct function of this heating current. Indirectly heated cathodes are also known and used; one suitble form of indirect heating is to direct at the cathode a heating electron beam, in which case the beam current of the heating beam can be adjusted to vary the cathode temperature. In any case, the temperature of the cathode can be varied by varying a "heating current" of some kind.
The useful operational life of the cathode is largely dependent upon the level of the vacuum in the electron gun and upon its operating temperature, and is shorter the higher the pressure and operating temperature are. The optimum temperature of a cathode for obtaining an extended operational life-time is near the lower limit of the socalled space charge region, i.e. the temperature at which the cathode emits somewhat more electrons than can be withdrawn by the accelerating field of the gun, and a negative space charge just begins to build up before the surface of the cathode. Below this temperature the cathode operates in the so-called saturation region, in which all of the emitted electrons are withdrawn by the accelerating field of the gun.
It is known to determine the optimum operating temperature of the cathode by measuring the heating current/emission current characteristic curve which has a bend at the transition from the saturation region to the space charge region (this will be discussed in greater detail with reference to FIG. 1). By repeating this process of establishing the optimum operation temperature of the cathode after some interval of time, the heating current will be set to successively lower values according to the burning down of the cathode as a result of evaporation and atomisation.
Because the vacuum in the region of the cathode is not normally constant, the heating cannot be reset depending upon the time of operation. A direct measurement of the cathode temperature would be difficult and costly.
In order to obtain reproducible results, electron beam machines usually operate with stabilised voltage and stabilised beam current. In a known process for setting the cathode heating current on the basis of the characteristic curve of the cathode heating current/cathode emission current the beam current regulation is disabled and the electron gun is operated at a constant Wehnelt voltage. The cathode heating current is varied in a particular range and the corresponding change of the emission or beam current is registered. Since the beam current changes during this process, the process cannot be carried out during a working operation, e.g. a welding operation. In order to avoid damage to the workpiece or its support a massive beam collector must be introduced into a path of the beam during measurement of the cathode characteristic curve.